1. Field of the Invention
The present invention relates to heat exchangers. More particularly, the present invention relates to high temperature heat exchangers.
2. Description of the Related Art
Presently known heat exchanger systems cannot operate efficiently with dirty and corrosive flue gases. One such known system is referred to as the dual fluidized bed waste heat recovery system. In this dual bed system, a "hot" fluidized bed is mounted above a "cold" fluidized bed. The air distributor of the hot fluidized bed is subject to a harsh environment in which the high temperature combustion gases cause severe corrosion, fouling, and warping. This system has many disadvantages relating to reliability concerns and high capital costs. Also, this dual bed system has been shown to have limited efficiency with respect to heat transfer before the flue gases are exhausted into the atmosphere.
In an effort to improve upon the dual bed system, a raining bed waste heat recovery system was devised in which the hot fluidized bed was replaced with a "raining bed" where hot combustion gases flow upwards against solid particulates discharged from a cyclone separator. These solid particulates eventually settle on the bottom of the raining fluidized bed. Although this system eliminated some of the problems associated with the dual bed system, there remain a number of problems relating to limitations on efficiency, high capital costs, and poor reliability due to a complicated system of solids recirculation.
Presently known heat exchangers are not adequate for removing heat from high temperature fluids containing contaminants. The contaminants often foul or corrode the heat exchanger, thereby decreasing its reliability and efficiency. A heat exchanger that can satisfactorily remove heat from a fluid containing contaminants is particularly needed, for example, to increase the efficiency of industrial processes. Often an industrial process will waste a significant amount of the thermal energy injected into and generated by the process when it expels high temperature waste gases to the atmosphere. A process expelling waste gases at temperatures of 2000.degree. F. may be losing more than 55 percent of the thermal energy injected into and generated by the process. The cumulative effect of such efficiency losses is significant. A tremendous savings could be realized if a heat exchanger existed that would more effectively and reliably recover heat from the waste gases.
An alloying process used in the secondary aluminum industry exemplifies the problem that contaminants in the waste gases of an industrial process present to known heat exchangers. In this process, magnesium is the primary contaminant in scrap aluminum. Magnesium is removed in a smelting furnace by bubbling chlorine through melted aluminum to form magnesium chloride, which is skimmed from the surface of the aluminum. However, some of the chlorine escapes into the waste gases and severely corrodes known metallic heat exchangers. Thus, no satisfactory heat exchanger is presently available to recover heat from the over 2000.degree. F. waste gases produced by aluminum smelting furnaces. Similar problems exist in other industries, such as titanium pigment production, formaldehyde production, phosphoric acid production, mineral wood production, and glass melting.
The high temperature heat exchanger can also find an application in the Integrated Gasification Combined Cycle (IGCC) systems and be used to reduce temperature before high temperature coal and flue gas filters. This would reduce the overall power plant capital cost and increase its reliability.
A heat exchanger is needed that can adequately recover heat from waste gases in industrial processes. Further, a heat exchanger is needed that can recover heat from a wide range of contaminated gases without being fouled or corroded. Also, there is a need for a low-cost heat exchanger that can operate under the described conditions.